Hydrothermal electrolysis apparatuses that oxidatively decompose reducing substances, such as organic substances (including synthetic polymers) and ammonia, by performing electrolysis under high-temperature and high-pressure conditions to cause hydrothermal reaction simultaneously are known (Patent Literature (PTL) 1 to PTL 3).
When electrolysis is performed with such an electrolysis apparatus, the temperature and pressure inside the reactor are increased. Therefore, the treatment of a liquid waste may be performed by a batch treatment or by a quasi-continuous treatment in which a batch treatment is repeatedly performed. However, it is difficult to treat a large amount of liquid waste by a batch treatment or a quasi-continuous treatment, because the amount of liquid waste that can be treated by one batch treatment or one quasi-continuous treatment is small.
PTL 3 proposes a hydrothermal electrolysis apparatus including cylindrical metal electrodes which is capable of continuously treating a large amount of wastewater. The hydrothermal electrolysis apparatus proposed in PTL 3 includes a cylindrical reactor, a flanged lid with which the cylindrical reactor is hermetically sealed, and a number of cylindrical metal electrodes disposed in the reactor which serve as internal electrodes.
The hydrothermal electrolysis apparatus proposed in PTL 3 has the following disadvantages.
i) Since a number of cylindrical metal electrodes are disposed in the reactor, the reactor is required to have a large inside diameter.
ii) The reactor is hermetically sealed with a flanged lid having a larger diameter than the reactor. This increases the size and weight of the apparatus.
iii) The reactor included in the apparatus needs to have a large thickness in order to enhance the pressure resistance of the apparatus. An increase in the thickness of the reactor leads to an increase in the heat capacity of the reactor, and the amount of energy required for heating the reactor is increased accordingly. The flanged portion, which is formed in the lid as described above, also has a large heat capacity. Therefore, a large amount of energy is required for performing heating in the initial stage.
iv) Since the flanged portion is formed in the lid, the surface area of the apparatus is increased accordingly. This also increases the amount of heat lost by heat dissipation.
v) In common electrolysis reactions, decomposition of water involves formation of bubbles. Although the formation of bubbles is suppressed when electrolysis is performed under high-temperature, high-pressure conditions, it is not possible to prevent the formation of bubbles. If bubbles are adhered to the surfaces of the electrodes, the contact efficiency between the liquid that is to be treated and the electrodes may be reduced, which results in, for example, a reduction in the reaction efficiency or an increase in the electrolysis voltage. Although the bubbles can be washed away and removed by increasing the flow rate of the liquid that is to be treated, it is difficult to remove bubbles adhered on the surfaces of the electrodes because, in the structure employed in PTL 3, channels have a large cross-sectional area and the velocity (linear velocity) at which the liquid flows along the surfaces of the electrodes is low even when the flow rate of the liquid is high.
vi) Since the electrolysis apparatus includes metal electrodes, the efficiency with which organic substances are electrolyzed is low.
vii) When metal electrodes are used, it is not possible to employ a bipolar structure under high-temperature, high-pressure conditions, and increasing the area of internal electrodes included in one cylindrical tube requires installing cathodes and anodes separately. This increases the complexity of the structure.
Electrolysis apparatuses used for performing electrolysis which include a bipolar electrode in order to simplify wiring and increase the current efficiency are known (PTL 4). In PTL 4, one or plural bipolar electrodes are interposed between an anode and a cathode so as to be parallel to the anode and the cathode. Upon a voltage being applied between the anode and the cathode, polarization occurs in the bipolar electrodes, which are not connected to a power source. This contributes to electrolysis. It is described in PTL 4 that conductive diamond electrodes are used as an anode, a cathode, and bipolar electrodes. Conductive diamond electrodes are electrodes including diamond doped with boron or nitrogen so as to have electrical conductivity. Self-supported conductive diamond electrodes produced by forming conductive diamond on a substrate and subsequently removing the substrate and conductive diamond electrodes produced by forming a conductive diamond film on a substrate composed of silicon or the like are used.    PTL 1: Japanese Patent No. 3970458    PTL 2: Japanese Patent No. 4002358    PTL 3: JP 2000-233186 A    PTL 4: JP 2004-237165 A